MCQs

A non-competitive inhibitor binds to a site different from the active site (an allosteric site). This binding alters the three-dimensional shape of the enzyme, including the active site, so the substrate can no longer bind effectively, regardless of substrate concentration.

Changing one amino acid (primary structure) can disrupt the local folding (secondary), which in turn alters the overall 3D shape (tertiary) and its ability to bind with other subunits (quaternary). Thus, all higher levels of structure are ultimately dependent on the primary sequence.

Tertiary structure is the overall 3D conformation of a single polypeptide chain, driven by interactions between the R-groups. This includes hydrophobic interactions, ionic bonds, hydrogen bonds, and disulfide bridges. The backbone H-bonding defines secondary structure.

Both α-helices and β-pleated sheets are secondary structures stabilized by regular hydrogen bonding between the backbone atoms (the C=O of one amino acid and the N-H of another). R-group interactions define the tertiary structure. Disulfide bridges are covalent, not hydrogen, bonds.

Non-essential amino acids are those the human body can synthesize de novo. Alanine can be produced from pyruvate. Lysine, phenylalanine, and valine are essential amino acids that cannot be synthesized and must be obtained from the diet.

The bulk of a lipid molecule, like a fatty acid or triglyceride, consists of long hydrocarbon chains (C-H bonds). These bonds are non-polar and hydrophobic, repelling interaction with polar water molecules and leading to insolubility.

A triglyceride has glycerol esterified to three fatty acids. A phospholipid is a modified triglyceride where one fatty acid chain is replaced by a highly polar phosphate group, which is often further linked to a nitrogenous compound, creating an amphipathic molecule.

The covalent linkage in the sugar-phosphate backbone of nucleic acids is a phosphodiester bond. It is formed between the 3' carbon of one sugar and the 5' phosphate group of the adjacent sugar. Glycosidic bonds link sugar to base, and peptide bonds link amino acids.

A reducing sugar has a free aldehyde or ketone group that can reduce (donate electrons to) another compound, such as Cu²⁺ to Cu⁺ in Benedict's test. Glycosidic bond formation masks this group. Polymerization is a separate property, and glucose is lipid-insoluble.

While organic molecules are the focus of discussion, water is the most abundant molecule constituting 70-90% of the cell's mass. Among organic biological molecules, carbohydrates like cellulose are the most abundant, but the question specifies "in the living world," making water the correct overarching answer.

Chitin is a linear polysaccharide of N-acetylglucosamine monomers, linked by β-glycosidic bonds, providing structural support in arthropod exoskeletons and fungal cell walls. Cellulose is the structural polymer in plant cell walls. Starch and glycogen are energy storage molecules.

A competitive inhibitor structurally resembles the substrate and competes for binding at the enzyme's active site. This effect can be overcome by increasing substrate concentration. It does not bind to the allosteric site or permanently alter the enzyme.

Unlike the rigid lock-and-key model, the induced fit model proposes that the active site is flexible. The initial substrate binding induces a conformational change in the enzyme, molding the active site into a precise complementary shape around the substrate.

Waxes are hydrophobic lipids that form impermeable coatings. In plants (e.g., cutin on leaves) and animals (e.g., sebum on skin/fur), their main function is to prevent water loss and provide protection, not energy storage, which is the role of fats and oils.

The active site is a 3D pocket formed by amino acid residues brought together via the protein's tertiary folding. It is complementary to the substrate's shape and chemistry, and models like "induced fit" show it is flexible, not rigid.

Unsaturated fatty acids contain kinks due to double bonds, preventing tight packing of the hydrocarbon tails. This increased space between lipids makes the membrane more fluid and permeable compared to a membrane rich in straight-chained saturated fatty acids.

Denaturation unfolds a protein by disrupting the non-covalent interactions (hydrogen bonds, ionic bonds, hydrophobic interactions) that stabilize secondary, tertiary, and quaternary structures. The primary structure's covalent peptide bonds usually remain intact.

The model proposes a dynamic, fluid phospholipid bilayer where individual lipid molecules can move laterally. Proteins are not just on the surface but are integral or peripheral, creating a "mosaic" pattern that floats within the fluid lipid sea.

A conjugated protein (holoprotein) consists of a protein part (apoprotein) and a non-protein part (prosthetic group). If the prosthetic group is a cofactor and the protein is an enzyme, its removal yields an inactive apoenzyme. The term specifically relates to the loss of the non-protein component.